Al-Maliki has said he sees no need to set a timetable but his government are working to build up Iraq's security forces as quickly as possible so US-led forces can leave.

The Sadrists ended a two-month boycott of parliament in January after pulling out in protest over the timetable issue and a meeting between Maliki and Bush.

Abdul-Mehdi al-Muteyri, a senior official in thes movement, said Sadr had ordered the pullout, saying al-Maliki was hamstrung by political parties in his government pulling him in different directions.

"We don't believe in partisan quotas. Under the direct orders of Moqtada al-Sadr we have decided we are going to leave the government in order to give the prime minister the best possible options so that he can run his government," Muteyri said.

The move is unlikely to bring down the government, but it could create tensions in the fractious Shia-led coalition.